Hi Sebastien,
Thanks for your question.
It's no problem for you to be the director of the trustee company. Section 201A of the Corporations Act 2001 (Cth) requires a company to have at least one director and that the director must ordinarily reside in Australia. There is no condition around citizenship or visa status - provided you ordinarily reside in Australia you can be the director.
Just keep in mind that if you start working for the company, it will be like any other employment relationship so you will need to make sure this is allowed under your visa. Usually this is allowed when you are here with a spouse who is on a 457 visa, but you can confirm this by checking your visa conditions or contacting the Department of Home Affairs:https://www.homeaffairs.gov.au/about/contact/make-enquiry
I hope this helps. Good luck with your new business.